A Breath of nothing

Member

Arrietty is a borrower and just 5 to 6 inches high, she was worried about someone steal away her famliy again.
When she heard a faint scrape on the door - a breath of nothing. Neither moved: it might be a passing vole.
Does it mean she feel relax to breath, because there acturally is nothing happened. or does it mean she is holding her breath?